A typical procedure for extending and completing or lining a wellbore is as follows. In this procedure it is assumed that an existing casing is already installed in the wellbore, with a tubular member being provided for hanging off the existing casing once the wellbore has been extended. The tubular member may be a completion screen or slotted liner, or any other such member, but for the sake of simplicity the tubular member will be referred to here as a liner. The casing may be a surface casing.
A drill bit on a drill string is used to drill a hole section so as to extend the wellbore to reach a target depth. The drill string is then pulled back to the previous casing shoe. A short trip is then typically performed to determine the hole condition before the completion string can be run in. The drill string is then pulled completely out of the hole, and the liner is then run into the hole on a completion string. When the completion string has reached the bottom of the hole (or a desired depth), the liner is hung off the existing casing.
However, the present applicant has appreciated the following issues with the above-described procedure.
Such a procedure is time consuming. A tripping out operation is required to pull out the drill string, with the liner then being assembled and a tripping in operation performed to run the liner into the borehole. This procedure can typically take two to five days.
Such a procedure also brings with it certain operational risks. The tripping out and tripping in can subject the formation to hydraulic forces, with the risk of formation instability occurring, and problem such as collapse or caving within the hole. If the hole condition is determined to be inadequate, for example in the case of instability resulting in collapse or caving, it may become necessary for the liner to be set before the target depth has been reached; the liner may become stuck and not be landed at the desired depth. The mechanical properties of a typical liner mean that it has insufficient strength to be rotated or tensioned to remove the hole collapses or caving materials during installation.
In addition, because of the length of time required for the tripping out and tripping in operations, the hole can be left open for some time; this leads to an increased risk of further formation instability, thereby increasing the risk of collapse or caving.
The present applicant has appreciated the desirability of addressing these issues.